The present invention relates to a process for the preparation of functionalized polymers which comprise at least one double bond, have functional groups containing a carbonyl radical along their chain and can be used in tires.
Since fuel saving and the need to protect the environment have become priorities, it has been desirable to produce mixtures of dienic polymers possessing the lowest possible hysteresis, which enables such polymers to be used in the form of semifinished products that form part of the composition of tires, such as underlayers, rubbers for bonding between different kinds of rubber, calendering rubbers for metal or textile reinforcements, and sidewall rubbers or treads. These polymers impart improved properties to tires, especially a reduced running resistance and an improved wear resistance.
It is known that the interactions between the polymers and the reinforcing fillers, based on silica or carbon black in the majority of cases, which are used in these tires have a significant influence on the above-mentioned properties of these tires.
To optimize these interactions, attempts have been made to modify the nature of the dienic polymers after polymerization, especially through the use of agents for functionalizing these polymers along their chain or at the end of the chain.
Several processes have been used heretofore for functionalizing an unsaturated polymer along its chain.
For example, the polymer may be functionalized by hydrosilylation, which consists essentially in reacting a solution of unsaturated polymer with a hydrogenated silicon derivative. Alternatively, the polymer may be functionalized by the grafting of azocarboxylate derivatives or by metallation using butyllithium.
For a detailed description of these processes, reference may be made to the following article: D. N. Schulz, K. N. Turner and M. A. Golub, Rubber Chemistry and Technology, 1982, vol. 55, pp. 809-859, incorporated herein by reference.
However, major disadvantage of these processes of functionalization along the chain is the relatively high cost they entail in order to obtain functionalized polymers which do not exhibit a change in macrostructure when compared to the starting polymers.
The object of the present invention is to overcome this disadvantage.
The present invention provides a process for the preparation of functionalized polymers comprising at least one double bond and having functional groups containing a carbonyl radical along their chain which involves:
subjecting a starting polymer comprising at least one double bond, in an inert hydrocarbon solvent, to a hydroalumination or carboalumination reaction along its chain by adding to said starting polymer an agent derived from aluminum,
adding to the product of this reaction at least one electrophilic agent that reacts with said agent derived from aluminum, and
subsequently stopping the functionalization reaction of the second step and recovering the polymer functionalized along its chain.
The starting polymer can be natural or synthetic and it may or may not already be functionalized. It can be a homopolymer or a copolymer, the term xe2x80x9ccopolymerxe2x80x9d covering polymers which are each obtained from two or more than two types of monomers, for example terpolymers.
Isoprene, butadiene, isobutylene and vinylaromatic compounds, which may or may not be substituted, are examples of monomers which can be used to obtain this starting polymer.
Polybutadiene or polyisoprene, for example, are among the starting homopolymers which can be used.
Starting copolymers which can be used, include, inter alia, styrene/butadiene copolymers obtained by anionic or emulsion polymerization, isoprene/butadiene copolymers, styrene/butadiene/isoprene terpolymers or terpolymers of ethylene, propylene and a diene.
The starting polymers used are advantageously dienic elastomers in order to obtain elastomers functionalized along the chain.
To carry out the hydroalumination or carboalumination reactions involved in the first step of the process, which amount to the addition of an Alxe2x80x94H or Alxe2x80x94C bond, respectively, onto a double bond of said starting polymer according to the equations:
Alxe2x80x94H+Cxe2x95x90Cxe2x86x92Hxe2x80x94Cxe2x80x94Cxe2x80x94Alxe2x80x83xe2x80x83(Eq. 1)
or
Alxe2x80x94C+Cxe2x95x90Cxe2x86x92Cxe2x80x94Cxe2x80x94Cxe2x80x94Alxe2x80x83xe2x80x83(Eq. 2)
an alkyl-aluminum compound or an aluminate can be used in particular as said agent derived from aluminum.
Preferably diisobutylaluminum hydride (hereafter xe2x80x9cDiBAHxe2x80x9d) is used as the agent derived from aluminum.
This first step is performed in an inert hydrocarbon solvent in such a way that the number of moles of agent derived from aluminum per 1000 g of starting polymer is between 0.05 mole and 5 moles, preferably between 0.05 mole and 0.5 mole.
Toluene, xylene, heptane or cyclohexane are preferred inert hydrocarbon solvents.
The first step is preferably carried out at a temperature of between 20xc2x0 C. and 100xc2x0 C., more preferably between 50xc2x0 C. and 70xc2x0 C.
In order to carry out the second step of the process according to the invention, an electrophilic agent containing a heteroatom, such as nitrogen and/or oxygen, is preferably used. Electrophilic agents which can be used include anhydrides, especially carbon dioxide, isocyanates or carbonyl derivatives.
In order to obtain a polymer with carboxylic acid groups along the chain, it is preferred that an anhydride, preferably carbon dioxide, is used as the electrophilic agent. It is also possible to use a cyclic anhydride such as succinic anhydride.
In order to obtain polymers with amide groups along the chain, it is preferred that an isocyanate, such as phenyl isocyanate, be used.
In order to obtain polymers with carboxylic acid or amide groups along the chain, the second step is advantageously carried out in such a way that the molar ratio of the electrophilic agent (in moles) to the agent derived from aluminum (in moles) is equal to or greater than 3.
To specifically obtain polymers functionalized with amide groups using phenyl isocyanate, said molar ratio is approximately equal to 4.
The second step is preferably carried out at a temperature of between 20xc2x0 C. and 100xc2x0 C., more preferably between 50xc2x0 C. and 70xc2x0 C.
The functionalization reaction of the second step is stopped, preferably by adding a metal complexing agent, which also has the effect of fluidizing the reaction medium, i.e. reducing viscocity. The complexing agent is preferably a metal chelate capable of releasing at least one proton in the complexation reaction.
Acetylacetone is preferably used as the chelating agent. Alternatively, benzoylacetone or 8-hydroxyquinoline may be used.
The molar ratio of the complexing agent (in moles) to the agent derived from aluminum (in moles) is then equal to or greater than 3.
In the case of functionalization with carboxylic acid or amide groups using carbon dioxide or phenyl isocyanate, respectively, as the electrophilic agent, a strong protonic acid, e.g. hydrochloric acid, is added to the reaction medium, following the addition of said metal complexing agent, in order to completely stop the functionalization reaction.
The molar ratio of the strong protonic acid (in moles) to the agent derived from aluminum (in moles) is then equal to or greater than 3.
The advantages of the present invention, can be more understood more clearly from the following non-limiting Examples which are provided for illustration and should not be construed as limiting the invention.
In these Examples the number-average molecular weights (Mn) of the starting polymers and the corresponding functionalized polymers were precisely determined by osmometry.
Also, the size exclusion chromatography (SEC) technique was used to determine the molecular weight distributions of samples of these polymers. Using standard products whose characteristics are described in Example 1 of European patent document EP-A-692 493, this technique made it possible to evaluate, for a sample, a number-average molecular weight (Mn) which has a relative value, as distinct from that determined by osmometry, and a weight-average molecular weight (Mw). The polydispersity index (Ip) of this sample was deduced therefrom (Ip=Mw/Mn).
In the SEC technique, the macromolecules are physically separated, using columns packed with a porous stationary phase, according to their respective sizes in the swollen state. Prior to this separation, the polymer sample is solubilized at a concentration of about 1 g/l in tetrahydrofuran.
The above-mentioned separation is carried out using a model xe2x80x9c150Cxe2x80x9d chromatograph marketed under the name xe2x80x9cWATERSxe2x80x9d. The eluting solvent is tetrahydrofuran, the flow rate is 1 ml/min, the temperature of the system is 35xc2x0 C. and the analysis time is 30 min. A set of two xe2x80x9cWATERSxe2x80x9d columns of the xe2x80x9cSTYRAGEL HT6Exe2x80x9d type is used.
The solubilized of polymer sample is injected in a volume of 100 xcexcl. The detector is a xe2x80x9cWATERSxe2x80x9d model xe2x80x9cR401xe2x80x9d differential refractometer. Also, a software with the trade name xe2x80x9cWATERS MILLENIUMxe2x80x9d is used to process the chromatographic data.